Toothed belt drives are advantageously adapted for providing positive power transmission free of slip, with relatively low noise. A common form of such toothed belt drive is one wherein the belt tooth has a trapezoidal cross section and the pulley groove is complementary thereto. A number of belt drives have been developed utilizing rounded teeth.
In the conventional toothed belt drive system, the belt tooth has a maximum backlash at the root portion thereof. While such a belt smoothly engages the pulley, such belts tend to wear rapidly and because of the possible movement of the belt relative to the pulley at start-up, the pulley tends to scrape the surface of the belt tooth, further aggravating the wear problem.
One solution to this problem has been to apply a substantial tension to the belt so as to minimize the movement of the belt relative to the pulley at start-up. It has been found, however, that the application of high tension to the belt causes the belt to lengthen and have shortened useful life due to early breakage thereof.
Another attempted solution to the problem has been to provide a toothed belt drive wherein there is a constant clearance between the belt tooth and the pulley groove sidewall from the belt tooth root to the belt tooth tip. This attempted solution has not proven fully satisfactory because of the interference with the pulley by the belt teeth, requiring the belt tension to be reduced. With reduced belt tension, the belt tends to jump from the pulley grooves under low loads.
In one form of prior art belt, the power transmission surface portion of the belt tooth flank is defined by an arc centered on the belt pitch line. Such design, however, causes the teeth to be relatively small so as to reduce the force transmission capabilities and thereby aggravating the belt jumping problem.
Another attempted solution has been to locate the center of the power transmission surface arc on the belt land line. This results, however, in the inclination of the belt tooth power transmission surface to become relatively large, thereby increasing interference of the belt with the pulley and requiring an increased backlash at the belt tooth root, thereby again causing the problem of movement of the belt relative to the pulley during start-up or upon reverse rotation of the drive.
The various problems cause a reduction in the accuracy in the positive driving by such toothed belts and is a vexatious problem which has not been fully solved, notwithstanding the longstanding need for such a solution.